dt-bindings: snps,dw-apb-ssi: Add mandatory clock bindings documentation

The Synopsys SSI driver uses a mandatory clock that is not documented,
so detail it in the device tree bindings. Also correct the spelling of
"pins" in the "Optional Properties" section for the driver.
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Gareth Williams <gareth.williams.jx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.txt

@@ -8,9 +8,10 @@ Required properties:
 - interrupts : One interrupt, used by the controller.
 - #address-cells : <1>, as required by generic SPI binding.
 - #size-cells : <0>, also as required by generic SPI binding.
+- clocks : phandle for the core clock used to generate the external SPI clock.
 
 Optional properties:
-- cs-gpios : Specifies the gpio pis to be used for chipselects.
+- cs-gpios : Specifies the gpio pins to be used for chipselects.
 - num-cs : The number of chipselects. If omitted, this will default to 4.
 - reg-io-width : The I/O register width (in bytes) implemented by this
   device.  Supported values are 2 or 4 (the default).
@@ -25,6 +26,7 @@ Example:
 		interrupts = <0 154 4>;
 		#address-cells = <1>;
 		#size-cells = <0>;
+		clocks = <&spi_m_clk>;
 		num-cs = <2>;
 		cs-gpios = <&gpio0 13 0>,
 			   <&gpio0 14 0>;